a. Field of Invention
This invention relates to an optical proximity sensor having two bifurcated optical bundles used in a servo loop.
b. Description of the Prior Art
Bifurcated optical fibers are frequently used in sensors to detect a distance to a target. Typically, one sensor bundle comprises two sets of fibers, each set originating from a respective arm, the two sets being joined in a common leg. Light is transmitted from a light source through one arm to the target and the corresponding light reflected from the target is conducted by the other arm to a light sensor. The intensity of the reflected light is indicative of the distance between the end of the common leg and the target. However, a direct measurement of the intensity has proven to be an unreliable parameter because of long and short term variations in the intensity of the light source, and variations in the reflectivity of the target. Numerous schemes have been devised in the past to overcome these problems. However, these schemes has proven to be unreliable and/or very expensive to implement.
For example, U.S. Pat. No. 4,,269,512 to Nosler discloses an optical position monitor with a laser directed at a target (such as a log) and a detector assembly monitoring the log at an acute angle with respect to the laser. The detector assembly includes a carriage moving transversally with respect to the monitoring direction and housing two adjacent light sensors with optical fibers. This system is not capable of sensing very small distances.
U.S. Pat. No. 4,281,245 to Biogardh et al. discloses several configurations with two or more interconnected optical fiber bundles, an optical filter, and a switching mechanism for choosing between two light sources having different frequencies.
U.S. Pat. No. 4,220,850 to McEachern illustrates the use of two different photo arrays in an autofocusing apparatus.
There is a present need for an optical sensor capable of indicating a distance to a target which yields reliable, consistent and accurate results and is inexpensive to make.